At one time I had see someone on E-bay selling a car with one of those. First, you have to think of the safety aspect of having a fuel tank in a place that wasn't designed to have one. Second, you lose trunk space. Third, the more fuel you are carrying, the more weight. So your fuel economy will decrease. Plus, who wants to spend 20 minutes at the pump filling up 40 gallons of fuel.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section